Various apparatus and methods for creating a passageway to a blood vessel are readily available; these apparatus and methods often involve the use of a cannula. The conventional cannula has the role of securing a passageway into a blood vessel to permit either the release of substances from the body or the injection of substances into the body. The processes of introducing a cannula into the body is called cannulation. The term cannula has been chosen in discussing the invention. However, the term catheter could be substituted as they are equivalent terms in the context of this invention.
Cannulation of a blood vessel, such as a vein or an artery, is a particularly delicate process because of the danger of damage to the blood vessel such as puncture or abrasion caused by the application of excessive force. Therefore, a number of devices and methods of using such devices have been developed to cannulate blood vessels while minimizing damage to the blood vessel.
Two examples of known cannulation procedures are described in U.S. Pat. No. Re 31,855 to Osborne (the Osborne patent). The first cannulation procedure described in the Osborne patent involves multiple exchanges of elements inserted into a blood vessel. The procedure involves the direct insertion of a rigid, hollow needle into a blood vessel, passing a guidewire through the needle, withdrawing the needle, passing a cannula over the guidewire and withdrawing the guidewire leaving just the cannula in place. The second cannulation procedure described in the Osborne patent involves the insertion of a rigid needle and cannula simultaneously into a blood vessel, and withdrawing the needle leaving the cannula in place. The cannula may be removed from the patient by pulling tabs disposed on the cannula, causing the cannula to tear in a longitudinal direction as it is removed.
One particular problem encountered during cannulation arises when the blood vessel is diseased in some way thereby reducing the flow path through the blood vessel and preventing proper cannulation. One possible solution to this problem involves the introduction of a catheter into the blood vessel which can assist the diameter of the blood vessel in returning to its pre-diseased diameter thereby increasing the fluid flow through the blood vessel. One example of a device which increases the diameter of a blood vessel is described in U.S. Pat. No. 4,921,484 to Hillstead (the Hillstead patent). The device described in the Hillstead patent is a steel mesh balloon attached to a catheter. The balloon is inserted into a blood vessel and is advanced to a desired point within the circulatory system. Expansion of the balloon widens the diameter of the blood vessel, thereby widening the flow path through the vessel. The purpose of this device is to dilate a blood vessel within the circulatory system. Therefore, the device is not configured to provide and maintain a widened flow path into, out of and through a diseased or a normal blood vessel.
Another example of a device used to maintain a widened flow path through a blood vessel is an endovascular stent. Stents are often used at a distant site in the body as a substitution for a vascular graft (usually in the aorta). One example of an endovascular stent is found in U.S. Pat. No. 4,580,568 to Gianturco (the Gianturco patent). The stent described in the Gianturco patent is positioned in a passageway in the cardiovascular system after first being compressed in a sheath which retains the stent in a compressed state. After the stent is properly positioned within the passageway, the sheath is withdrawn, thereby allowing the stent to expand to its pre-compressed configuration. As long as the stent is maintained within the blood vessel, a cannula cannot be inserted through the area within the blood vessel containing the stent. Hence, a cannula cannot be used simultaneously with the stent to properly cannulate a blood vessel.